Current widely applied wireless communication techniques include no more than the 3rd generation (3G) mobile communication technique in which Code Division Multiple Access (CDMA) plays a leading role and the WiMAX wireless wideband access technique in which Orthogonal Frequency Division Multiple Access (OFDMA) plays a leading role.
The CDMA stands for a code division multiple access technique. Briefly, it horizontally divides a bandwidth into multiple spread spectrum—Because the However, if uplink code channels of the CDMA are asynchronous, the Inter-Symbol Interference (ISI) will be enlarged greatly. The Synchronous CDMA (SCDMA) solves the problem of non-o However, in the presence of severe multi-path interference, orthogonality of the code channels will also be destroyed. Although multi-user joint detection technique has some effect on the multi-path condition, the performance still decreases greatly in the condition that there are too many users and the multipath is complicated. Meanwhile, it is quite difficult to realize the multi-user joint detection technique. Therefore, the CDMA is not applicable for wideband applications.
The OFDMA stands for orthogonal frequency division multiple access. Briefly, it longitudinally divides the bandwidth into multiple quite narrow frequency points and then implements frequency division multiple access through allocating the frequency points to different users. As long as an enough time interval (cyclic prefix) is reserved, signals of different users hardly interfere with each other because the between the frequency points will not be affected by the multi-path delay. Thus, the multi-user joint detection may be divided into multiple independent single-user detections. However, because the OFDMA is born by narrowband frequency points, its amplitude fluctuates dramatically in the case of multi-path, which causes severe signal attenuation, particularly to narrowband voice communications. In addition, because each two adjacent cells with the same frequency adopt the same frequency points, it is basically impossible to counteract interferences between adjacent cells and it is difficult to realize co-frequency networking.